Because of their excellent coating operability and good anti-corrosion properties of the resulting coating films, cationic electrodeposition coating compositions are widely used as undercoating compositions for conductive metal products such as automobile bodies, which are required to have the aforementioned properties. In electrodeposition coating using cationic electrodeposition coating compositions, attempts are made to maintain the inner film thickness of bag portions and gap portions to ensure anti-corrosion properties, and to optimize or make the outer film thickness uniform (for example, to ensure the film thickness of a portion where finish and anti-corrosion properties are valued) for improving the quality of automobile bodies and reducing the cost.
However, in coating films obtained using conventional electrodeposition coating compositions, a reduction in thickness from 20 μm to 15 μm may result in a poor finish due to an uneven substrate and a reduction in heat flow properties. Further, for the purpose of improving finish, if the amount of plastic components used in the coating film is increased to improve heat flow properties during heating and drying, anti-corrosion properties tend to be lowered. Because of these factors, it was difficult to provide an electrodeposition coating film having both satisfactory finish and anti-corrosion properties at a dried film thickness of 15 μm.
In view of these circumstances, the development of a cationic electrodeposition coating composition that exhibits excellent electrodeposition coating applicability onto hot dip galvanized steel sheets in an article having complicated bag portions and gap portions; and that provides a cationic electrodeposition coating film having excellent anti-corrosion properties and a superior finish, in particular, a superior finish at a dried film thickness of 15 μm, has been desired.
Patent Literature 1 discloses a method for producing a coating film, the method being characterized in that, in the electrodeposition coating of a cationic electrodeposition coating composition, the electric quantity required for initiating the deposition of a coating film is 100 to 400 C/m2, and the polarization resistance value per unit film thickness is 50 to 300 kΩ·cm2/μm. This method considers various resin components.
Further, Patent Literature 2 discloses an anion electrodeposition coating composition comprising a carboxyl group-containing polyester resin and a cross-linking agent, the composition further comprising a resol phenolic resin to improve anti-corrosion properties.
However, the above findings cannot provide an electrodeposition coating film having excellent anti-corrosion properties and a satisfactory finish, particularly, a satisfactory finish at a dried film thickness of 15 μm.